If we hope to limit some of these climate change-related consequences, nations and industries must make drastic cuts — and soon — to greenhouse-gas emissions from energy production, transportation, industrial work, farming, and other sectors. An increasing number of people are demanding such action: In September, 4 million across 161 countries participated in a worldwide climate strike led by Swedish youth activist Greta Thunberg.

But if emissions continue to increase and Earth’s temperature increases by more than 3 degrees Celsius, according to the IPCC , oceans would be an average of 3 feet higher by the year 2100. Those rising seas would displace 680 million people in low-lying coastal zones, along with 65 million citizens of small island states.

Even if carbon emissions dropped to zero tomorrow, scientists say we’ll still be watching human-driven climate change play out for centuries.

„There’s no stopping global warming,“ Gavin Schmidt, a climate scientist and the director of NASA’s Goddard Institute of Space Studies, previously told Business Insider. „Everything that’s happened so far is baked into the system.“

Now it’s a matter of trying to „save what we can save,“ according to Thunberg.

Here’s what the Earth could look like by 2100 in our best- and worst-case scenarios.

What Earth might look like in 80 years if we’re lucky — and if we’re not slides

What Earth might look like in 80 years if we’re lucky — and if we’re not slides

To understand where we're headed, we must first assess the changes we're already observing. Since 2001, we've seen 18 of the 19 warmest years ever.

Skye Gould/Business Insider

Four of the five hottest years on record have happened since 2015. This year is on pace to be the third-hottest on record globally.

The planet's oceans absorb a whopping 93% of the extra heat that greenhouse gases trap in the atmosphere. Last year was the hottest year on record for the oceans.

Reuters/Rodrigo Garrido

That warming threatens coral reefs worldwide. Warm waters cause corals to expel the algae living in their their tissues and turn white in a process called bleaching. At present rates, it's expected that 60% of all coral reefs will be highly or critically threatened by 2030.

Warming waters are also melting Arctic and Antarctic glaciers from underneath. In 2012, Greenland lost more than 400 billion tons of ice, which was almost quadruple the amount of loss in 2003.

In the 1980s, Antarctica lost 40 billion tons of ice annually. In the last decade, that number jumped to an average of 252 billion tons per year.

Oceans also absorb more than one-third of the carbon dioxide in the atmosphere, which causes them to become more acidic. We are already seeing the effects of that elevated acidity in the form of coral bleaching.

A warming planet leads to more extreme weather, both cold and hot. A 2017 study found that the frequency of polar-vortex events has increased by as much as 140% over the past four decades.

Climate change is also linked to more warm, dry days in regions with a risk of wildfires, like California. In November 2018, the most deadly and destructive wildfire in the state's history — the Camp Fire — started during what is typically the rainy season.

As the climate warms, California's wildfire season is getting longer because the snow pack melts sooner.

Schmidt is fairly pessimistic about our ability to keep temperature-rise under the threshold scientists consider safe. "I think the 1.5-degree target is out of reach," he said, adding that we may blow past that by 2030.

Stephane Mahe/Reuters

He's more optimistic about keeping temperatures from rising more than 2 degrees Celsius, though. That's the upper threshold of temperature rise that the Paris climate agreement set to avoid at all costs.

Vincent Kessler/Reuters

So let's assume — optimistically — that we land somewhere between the targets of 1.5 and 2 degrees Celsius of temperature rise.

NASA

Even in that case, summers in the tropics would see a 50% increase in extreme-heat days by 2050.

Summer temperatures in the US will keep rising, and much of the western and central US will see a reduction of soil moisture, which exacerbates heat waves. By 2100, extreme heat days that typically happened once every 20 years are projected to occur every few years.

For example, the temperature in the Arctic Circle soared above freezing for one day in 2016. That's extraordinarily hot for that area, but such abnormalities would start happening a lot more.

If the world were to meet its most ambitious climate-change goals, average winter temperatures in the Arctic will still rise by up to 5 degrees Celsius by the year 2050.

REUTERS/Thomas Peter

In the summer of 2012, 97% of the Greenland Ice Sheet's surface started to melt. That's typically a once-in-a-century occurrence, but with 2 degrees Celsius of temperature rise, we could see extreme melt like that every six years by the end of the century.

That means years like 2016, which had the lowest sea-ice extent on record, would become more common. Summers in Greenland could become ice-free by 2050.

If warming continues, even at its current rate, a major glacier near the South Pole could reach a tipping point after which it loses all its ice. That loss could trigger a chain reaction of melting in the Antarctic called a "pulse," which could raise global sea-levels by 8 feet.

Even in our best-case scenarios, oceans are on track to rise 2 to 3 feet by 2100. This is because water expands as it warms (like most things). That could displace up to 4 million people. More dire scenarios suggest we'll see more than 700 million climate refugees displaced by rising seas.

About 17% of all the US's threatened and endangered species are vulnerable to rising sea levels and storm surges, including the Hawaiian monk seal and the loggerhead sea turtle.

Other human activities, such as deforestation and logging, could also devastate the planet's flora and fauna species. In roughly 50 years, 1,700 species of amphibians, birds, and mammals will face a higher risk of extinction because their natural habitats are shrinking.

Insects are especially at risk. The total mass of all insects on the planets is decreasing by 2.5% per year. If that trend continues unabated, the Earth may not have any insects at all by the next century. That's a major problem, because pollinators perform a crucial role in fruit, vegetable, and nut production.

If that all sounds scary, let's take a look at our worst-case scenario.

REUTERS/Gene Blevins

If greenhouse-gas emissions remain unchanged, we'll see more frequent and intense heatwaves like the ones Europe experienced this summer. In July, temperatures were up to 18 degrees Fahrenheit (10 degrees Celsius) above normal for parts of the continent.

Temperature records will continue to be broken. During Europe's July heat wave, parts of France hit 114 degrees Fahrenheit (45 degrees Celsius). Globally, that month was the hottest ever recorded on Earth.

Without curbing emissions, the tropics would stay extremely hot all summer long. In more temperate zones, 30% or more days would have temperatures that we currently consider unusual.

If we do nothing to curb greenhouse-gas emissions, roughly 5.3 million more acres are projected to burn each year in the US by 2100. That's an area the size of Massachusetts, and double today's burn rate.

Even a little bit of warming strains water resources. Left unchecked, climate change might cause severe drought across 40% of all land on the planet — double the amount today.

Not only do extreme temperatures and severe weather make farming more difficult, climate change is also expected to decrease the nutritional value of important food crops like wheat and rice. That's a dire threat for the 821 million people who are already undernourished worldwide, and it could cause a food-security crisis.

Researchers also worry that if we see more than a 2-degree-Celsius temperature increase, that could tip the balance of our planet's systems toward a "hothouse Earth" scenario. In that case, a feedback loop could lead temperatures to rise by 4 or 5 degrees Celsius.

In an extreme scenario, rising temperatures could even cause stratocumulus clouds to disappear entirely in the next 150 years. Without these clouds, which shade the planet from the sun's rays, the Earth's temperature could rise up to an additional 8 degrees Celsius.

Aleksandar Georgiev/Getty Images

Right now, humanity is on a precipice. We could end up with what Schmidt envisions as a "vastly different planet."

Reuters

Every fraction of a degree of warming that we can avoid, however, makes a difference.